1. Field of the Invention
The present invention relates generally to photolithographically patterned spring contacts, and more particularly to a plurality of such photolithographically patterned spring contacts vertically displaced from one another for use in electrically contacting integrated circuits, arrays, and the like.
2. Description of the Prior Art
Photolithographically patterned spring devices (referred to herein as “microsprings”) have been developed, for example, to produce low cost probe cards, and to provide electrical connections between integrated circuits. Such microsprings are disclosed and described, for example, in U.S. Pat. No. 5,914,218, which is incorporated by reference herein. A microspring is generally a micrometer-scale elongated metal structure having a free (cantilevered) portion which bends upward from an anchor portion which is affixed directly or indirectly to a substrate. The microspring is formed from a stress-engineered metal film (i.e., a metal film fabricated to have a stress differential such that its lower portions have a higher internal compressive stress than its upper portions) that is at least partially formed on a release material layer. The free portion of the microspring bends away from the substrate when the release material located under the free portion is removed (e.g., by etching).
The stress differential is produced in the spring material by one of several techniques. According to one technique, different materials are deposited in layers, each having a desired stress characteristic, for example a tensile layer formed over a compressive layer. According to another technique a single layer is provide with an intrinsic stress differential by altering the fabrication parameters as the layer is deposited. The spring material is typically a metal or metal alloy (e.g., Mo, MoCr, W, Ni, NiZr, Cu), and is typically chosen for its ability to retain large amounts of internal stress. Microsprings are typically produced using known photolithography techniques to permit integration of the microsprings with other devices and interconnections formed on a common substrate. Indeed, such devices may be constructed on a substrate upon which electronic circuitry and/or elements have previously been formed.
Such microsprings may be used in probe cards, for electrically bonding integrated circuits, circuit boards, and electrode arrays, and for producing other devices such as inductors, variable capacitors, scanning probes, and actuated mirrors. For example, when utilized in a probe card application, the tip of the free portion of a microspring is brought into contact with a contact pad formed on an integrated circuit, and signals are passed between the integrated circuit and test equipment via the probe card (i.e., using the microspring as an electrical contact).
Microsprings typically terminate at a tip, spaced apart from the substrate. In certain applications, the microspring has a tip profile (e.g., an apical point) capable of physically penetrating an oxide layer that may form on the surface to which electrical contact is to be made. In order to provide a reliable contact with a surface to be contacted, the microspring must provide a relatively high contact force (the force which the spring applies in resisting a force oppositely applied from the surface to be contacted). This is particularly true in applications in which the apical point must penetrate an oxide layer. For example, most probing and packaging applications require a contact force on the order of 50-100 mg between the tip and the structure being contacted.
One problem faced by typical microsprings is the tradeoff made between contact force and spring geometry. In general the contact force at the tip of the microspring is given by:
      F    tip    =                    wh        2            ⁢      Δσ              12      ⁢      x      where w is the width of the microspring, h is the thickness of the microspring, Δσ is the total stress difference vertically across the cross-section of the microspring, and x is the distance from the microspring tip to the anchor. Thus, there are several ways to increase the microspring's contact force, but at the cost of altering the microspring geometry.
First, as one decreases the length (X) of the microspring one increases its contact force. However, it is critical that when released the microspring tip should be at a certain height above the substrate. The spring must also provide a certain amount of compliance in response to a downward force being applied by the surface to be contacted. Too short a microspring produces problematic contact and inadequate compliance. Thus, there is a limit to the extent that one can reduce microspring length to increase the contact force.
Second, one can increase the microspring thickness (h) to increase the contact force. However, thickness also affects the extent of curvature resulting from a stress differential, thus again affecting tip height and compliance. Above a certain thickness a microspring is incapable of reaching the design requirement for tip height, as well as sufficient compliance.
Third, one can apply a plating material (e.g., Ni) over a microspring after its release. However, the plating process risks damage to the microspring, and deposits material underneath the microspring, potentially interfering with the motion of the spring. During the plating process, microsprings tend to adhere to the substrate surface, affecting device yield. Furthermore, in order to sufficiently increase the contact force, the thickness and width of the plated microspring increases, and may lead to the disadvantage of reducing the number of springs per mm in an array (i.e., decreasing spring pitch) as well as the disadvantages associated with excessive cross-sectional thickness and width discussed previously.
Furthermore, typical microsprings curve upwards and terminate at the apical tip. This tip, whether patterned into a point or a flat edge perpendicular to the long axis of the microspring, tends to dig into the contact point of the structure being contacted. While this has some benefit, for example when attempting to pierce an oxide layer, it is detrimental when there is some variability in the location of the contact point or need to accommodate small amounts of relative motion between the microspring and the point of contact. In the later case, there is a desire for a microspring with a tip profile capable of accommodating lateral tip movement, for example as it is vertically displaced, in order to maintain continued contact with the contact point.